1. Field of the Invention
The invention relates generally to the field of power converters controlled through digital pulse-width modulation (PWM). More particularly, the invention relates to a digital control scheme for PWM converters that uses non-linear gain scheduling to achieve a fast transient response while maintaining a slow response near the equilibrium point to ensure stability and reduce sensitivity to noise.
2. Description of Related Art
The use of digital pulse width modulation (DPWM) to control the output of a switching power converter is well known in the art. For example, FIG. 1 depicts a block diagram of a typical power conversion system in which the output 104 of a power stage 102 is controlled by a DPWM module 114. An error circuit 108 computes a difference between the output 104 of the power stage and a reference voltage 106. The error signal is digitized by an analog-to-digital converter (ADC) 110. The ADC output is filtered by a compensator circuit 112 that generally has a proportional-integral-differential (PID) character. The output of the compensator filter 112 then drives a digital pulse width modulator 114 that controls the switching cycles of the power stage 102.
One of the challenges faced by power converter designers is providing a fast response time while maintaining system stability against oscillation and minimizing overshoot. To address this issue, some designers employ non-linear methods. For example, FIG. 2 illustrates a non-linear control approach taken by some designers that comprises introducing a window comparator circuit 202 in parallel with the compensator filter 112. For small transients, the output of the compensator filter passes essentially unmodified to the DPWM for normal linear control operation. But when a large transient occurs, the window comparator 202 will notify the DPWM controller 114, which may make a decision to respond immediately before the end of the current switching cycle. While such a method may provide a fast transient response, it is essentially a hysteretic control approach, which makes it susceptible to stability problems. Accordingly, it would be desirable to provide a non-linear control approach that maintains a fast transient response while also suppressing oscillation and jittering around the steady state operating point.